1. Field
The present disclosure relates to a positive active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as a power source for small portable electronic devices. The lithium rechargeable batteries use an organic electrolyte solution and thereby, have a higher discharge voltage (e.g. twice as high) compared to a conventional battery using an aqueous alkali solution and accordingly, lithium rechargeable batteries have a higher energy density.
These rechargeable lithium batteries are used by injecting an electrolyte into a battery cell including a positive electrode, which includes a positive active material that can intercalate and deintercalate lithium, and a negative electrode, which includes a negative active material that can intercalate and deintercalate lithium.
As for the positive active material, LiCoO2 has been widely used but has a problem of capacity limitations and safety. Accordingly, development of an alternative material is actively being researched.
Since LiCoO2 has stable electrochemical characteristics, LiNiO2 has high-capacity, and LiMnO2 has good thermal stability and low cost, research on a Co—Ni—Mn three component-based lithium metal composite oxide (herein also referred to as “NCM” or “NCM composite oxide”) is made to combine these features.
The NCM has different characteristics depending on a composition ratio among Co, Ni, and Mn. For example, when Mn is included in a higher ratio, safety is improved, when Co is included in a higher ratio, an output characteristic is improved, and when Ni is included in a higher ratio, a capacity characteristic is improved.
Recently, a positive active material having high specific capacity in a rechargeable lithium battery for xEv for increasing energy density per unit weight and thus, increasing mileage, has drawn attention.
Accordingly, active research has been made on a Ni-rich positive electrode material prepared by including Ni in a higher content in a NCM, for example, NCM111 (30% of Ni) having a specific capacity of 150 mAh/g, NCM523 (50% of Ni) having a specific capacity of 160 mAh, NCM622 (60% of Ni) having a specific capacity of 170 mAh/g, NCM75105 (75% of Ni) having a specific capacity of 180 mAh/g, and NCM85105 (85% of Ni) having a specific capacity of 190 mAh/g. In other words, it can be seen that as a NCM includes Ni in a higher ratio, specific capacity of the active material is increased. However, when an active material includes Ni in a higher content, air in a firing (heat treatment) atmosphere should be controlled, which brings about a problem of increasing a manufacturing cost, and may lead to a content of remaining lithium such as Li2CO3 and the like. This remaining lithium may generate gas, and the generated gas may increase resistance of a battery and deteriorate its cycle-life. Additionally, when Ni is included in a higher content, cation mixing occurs and leads to increased capacity deterioration of a battery in terms of the structure of the active material. The ICP analysis of a negative electrode and an electrolyte in such a deteriorated battery cell shows severe elution of Ni. Accordingly, an electrolyte capable of decreasing of gas generation and controlling deterioration of cycle-life caused by the Ni-rich high-capacity positive active material is desired.